I've been losing weight for over half a year now just by exercising and eating healthy :) but now I weigh 52kg (114lbs) and I'm 1m72 (5'8) tall with is technically underweight (BMI 17,5) so I know I should stop losing weigh. But I have trouble allowing myself to eat more or eating 'junk' and I don't want to give up exercising because I'm training for my first marathon. Does someone has some advice for me?
Yeah, eat more. If you're running long distance you'll never get fat. Believe me, I've been into long distance running a long time now and when I go up 5-10lbs (which is hardly a visual difference other than my abs) I just run more and it's gone within 1-2 weeks.
I eat everything I come across and then just use exercise to balance the difference. You'll rarely gain even 0.5lbs in fat a day (you'd have to eat 4000+ calories to do that), so by that note you'll never gain more than 3lbs in a week even while indulging to your fullest. So eat more, and don't worry about it. Maintaining weight is easy.
I went from 119 to 103 (I'm 5'4) in a matter of 4 months. I never intended to lose weight, just wanted to tone up, but my caloric intake couldn't keep up with my cardio. I've gained 2 lbs back only by forcing extra food (I have no appetite and my stomach doesn't want it), eating a couple of days a week at McDonald's, and I've been drinking a protein shake with a meal. Unfortunately, I did have to cut back on the cardio, but, I've added weight training. I've not lost any more weight. Good luck.